1. Field of the Invention
The present invention relates to a differential limiting control apparatus for a vehicle installed between a front axle and a rear axle of a four wheel drive vehicle, particularly to a differential limiting a differential motion between rotational shafts of front and rear wheel.
2. Description of the Related Art
Conventionally, as a differential limiting control between one rotational shaft and the other rotational shaft, in particular, between front and rear axles of a four wheel drive vehicle, a technique has been known in which an actual differential speed between the rotational shafts is detected is caused to converge to a target differential speed by a feedback control through a PID (proportional Integration and Differentiation) control.
In addition, JP-A-9-109716 discloses a four wheel drive control apparatus for controlling such a clutch that a feedback control is implemented in which a torque distributed to front wheels is increased in accordance with a front and rear wheel speed differential between the rear wheel speed which are main driving wheels and the front wheel speed which are sub-driving wheels when a vehicle is normally driven, and a feed forward control is implemented in which a torque distributed to the front wheels is set in accordance with a throttle opening when the vehicle is started from rest.
In the aforementioned related art, however, the influence is not taken into consideration that is caused by the occurrence of a rotational speed differential triggered when there is a difference in a tire air pressure or tires which are different in diameter are mounted, and there is a probability that a slip control does not function properly due to the difference in the tire air pressure due to a diameter difference even when the feedback control is combined with the feed forward control, whereby the compatibility of stable vehicle behaviors and improved limit performance is made difficult to be attained. In addition, in case a tire which differs largely in the diameter from the others is mounted, an internal (or in-between)circulating torque increases, and depending upon conditions, there may be caused a risk that a reverse torque is generated on one or some of the tires.